Transcript I. Recessive Genetic Disorders

Complex Inheritance
and Human Heredity
Chapter 11
I. Recessive Genetic Disorders
A recessive genetic disorder – the
disorder is expressed when an
individual is homozygous recessive for
the trait.
Carrier – carriers are heterozygous for
the disorder so carry the gene but do
not express the disorder because it is
masked by the dominant alele.
1. Alkaptonuria
 This
disorder results in black urine!
 Caused by an enzyme deficiency.
 Acid excreted into the urine due to enzyme
deficiency.
 Discovered by Dr. Archibald Garrod (U.K.)
 He determined that this disorder runs in
families and is expressed in those who are
homozygous recessive.
2. Albinism
Genes don’t produce enough pigment –
called melanin.
 No color in skin, eyes, hair.
 Pink pupils, skin susceptible to UV
damage.
 1 in 17,000 (in the U.S.)

3. Cystic Fibrosis

Excess mucous in the lungs
and digestive tract
 Very susceptible to
infections
 Usually don’t live past
childhood
 1 in 3500 (in the U.S.)
4. Tay-Sachs Disease
 Fats
accumulate in the nerve cells in the
brain
 Brain deteriorates
 Blindness, deafness, and muscle atrophy
sets in
 Patients usually die by age 5
 Affects 1 in 2500 (in the U.S.) – usually
people of Jewish descent
Tay-Sachs Disease:
5. Galactosemia
 Lactose
intolerance
 Inability of the body to digest galactose.
lactose = galactose + glucose
 Galactose must be broken down into
glucose by GALT enzyme
 Those who lack or have defective GALT
cannot digest galactose and should avoid
milk products.
 Affects 1 in 50,000 to 70,000 (in the U.S.)
II. Dominant Genetic Disorders
 Some
disorders are caused by dominant
alleles.
 Expressed when at least one dominant
allele is present.
 Those who do not have the disorder are
homozygous recessive for the trait.
1. Achondroplasia
 Dwarfism
 Affects
1 in 25,000 (in U.S.)
 Cartilage in bones of arms
and legs slowly hardens
resulting in shorter than
normal arms, legs, and
fingers.
2. Huntington’s Disease
 Appears
between the ages of 30 and 50
years old
 Affects the nervous system
 Causes loss of brain function, muscle
spasms, and emotional disturbances
 Affects 1 in 10,000 (in the U.S.)
Codominance and Multiple Alleles:
ABO blood groups are examples of both.
Possible
Genotype
Blood Type
Phenotype
• Multiple Alleles - more
A
IAIA or IAi
B
IBIB or IBi
AB
O
I AI B
(codominance)
ii
than two possible alleles
for a specific trait (the
blood type gene - IA, IB
and i).
• Codominance in AB
blood type as both IA and
IB are expressed
simultaneously.
Sex Determination

46 chromosomes in humans
(23 pairs)
 Autosomes (22 pairs)
 Sex chromosomes (1pair)
XX = female XY = male

Whose gamete determines
the gender of the offspring?
Dosage Compensation

X Chromosome



Y Chromosome


Is larger than the Y
Carries genes necessary for
the development of both
females and males.
mainly carries genes for male
traits.
Dosage Compensation
(x-inactivation)
 one X in females is inactive.
This is a random event that
occurs in all mammals.
Example: The coat color of a calico cat is the result of the
random inactivation of one of the X chromosomes. In some
cells, the X chromosome that was inherited from the mother
is expressed, and in others, the X chromosome inherited
from the father is expressed.
http://www.bio.miami.edu/dana/dox/calico.html
Sex-Linked Traits
 Traits
controlled by genes located on the X
chromosome
 Males affected by recessive X-linked traits
more often than females. WHY?
Hemophilia – delayed clotting of blood.




A recessive sex-linked disorder
Affects more men than women
Hemophiliacs bruise easily. A small cut can
result in a huge loss of blood which can cause
death.
See Queen Victoria’s Pedigree on page 308
in your text book.
Pedigree
A
diagram that traces the inheritance of a
particular trait through several
generations.
= MALE
= FEMALE
A
AA
A
Pedigrees……
- are used to determine genotypes from
observed phenotypes.
- help genetic counselors determine
whether inheritance patterns are dominant,
recessive, or sex-linked.
- can be used to predict disorders in future
offspring.
Pedigrees can be used to trace genetic
disorders: shaded = affected individual
Hemophelia Pedigree
http://www.execulink.com/~ekimmel/drag_gr11/pedigree.htm
Complex Patterns of Inheritance
A. Incomplete Dominance – when the
heterozygous phenotype is an
intermediate phenotype between the two
homozygous phenotypes.
B. Codominance – both alleles are expressed
in the heterozygous condition.
 Sickle-cell Anemia


Red blood cells are
abnormally shaped.
Causes blood clots
and slow blood circulation.

All calico cats are female because coat color is
on the X chromosome.
 Since females have 2 X chromosomes, in all
cells one of the X chromosomes is turned off =
orange and black spots.
 Males have just 1 X chromosome so they can
only have one color of spots.
Sex-Linked Disorders
 1.

Colorblindness – a recessive x-linked
trait.
Affects 1 in 10 males (males have one X
chromosome so all traits on X chromosome
are expressed).
Polygenic Traits
 Phenotypic
traits that arise from the
interaction of multiple pairs of genes.




Skin color
Height
Eye color
Fingerprint patterns
Environmental Influences
 Can



have an effect on phenotype.
Heart disease – diet and exercise
Plant growth and survival – sunlight and water
A Siamese cat’s fur color!! – temperature affects
gene expression
Nondisjunction:
A. Normal Meiosis
B. Failure of homologous
chromosome pair to separate during
Meiosis I
C. Failure of sister chromatids to
separate during Meiosis II
Chromosome Disorder
 Down’s
syndrome = extra
copy of chromosome 21
 Called trisomy 21
 Chromosomes fail to
separate during meiosis
 Affects 1 in 800 babies
Trisomy 21 – Down’s Syndrome
Normal karyotype:
Sex Chromosome Disorders
 Only
one X chromosome = sterile female
because sex organs don’t develop
(Turner’s syndrome)
 Extra X chromosomes (xxy or xxxy) result
in sterile males
 Cannot have just a y – would be fatal